click.js

var simulateMouseEvent = function(element, eventName, coordX, coordY) {
  element.dispatchEvent(new MouseEvent(eventName, {
    view: window,
    bubbles: true,
    cancelable: true,
    clientX: coordX,
    clientY: coordY,
    button: 0
  }));
};

var simulateClick = function(selector) {
  var element = document.querySelector(selector);
  var box = element.getBoundingClientRect();
  var coordX = box.left + (box.right - box.left) / 2;
  var coordY = box.top + (box.bottom - box.top) / 2;

  simulateMouseEvent (element, "mousedown", coordX, coordY);
  simulateMouseEvent (element, "mouseup", coordX, coordY);
  simulateMouseEvent (element, "click", coordX, coordY);
}

simulateClick("#textboxButton");